Pellet stoves are operated in many domestic, commercial, and industrial premises as a heating source. Pellet stoves burn fuel in the form of pellets. The pellets are made of ground up wood, which is dried to a low moisture content (approximately 8%), and then pressed into pellet shapes. The pellets are typically approximately 0.25 inches in diameter, and may range up to about one inch long, though other pellet sizes are possible. The pressing process creates heat that brings the natural resin of the wood to the outer shell of the pellets. This natural resin is the bonding agent that maintains the ground up wood in a pellet shape.
There are hardwood and softwood pellets with different grades of pellets in both hardwood and softwood. Hardwood pellets typically create more ash and do not burn any hotter than softwood pellets. Lesser grades of pellets usually create less heat, more ash, and more pollution.
For pellet fuel to burn completely with the least amount of emissions, air must be passed over the burning pellets to create enough heat to penetrate the natural resin located on the outside of the pellet. If insufficient air is passed over the burning pellets, the burn temperature drops, the pellets smolder and smoke, and the pellets may eventually be extinguished.
Pellet stoves have many advantages over conventional wood stoves and other heating sources. The primary advantage is their low emissions. The U.S. Environmental Protection Agency (EPA) has issued new regulations for wood burning stoves, and many existing wood burning stoves will not comply with the new regulations. In addition, many local government entities are trying to reduce dramatically the number of wood stoves due to pollution concerns.
Pellet stoves fall into two main categories, namely, electrically powered pellet stoves and natural draft gravity-fed pellet stoves. An electrically powered pellet stove uses an electrically powered auger to transport the pellets from a hopper to a burn chamber. Being powered, such stoves can utilize additional powered devices such as fans to ensure sufficient air flow. Powered pellet stoves have several disadvantages, as compared with gravity-fed systems, including the noise of the fan and auger motor, the cost of replacement parts, down time to replace components, and the inability to work during power outages.
A natural draft gravity-fed pellet stove includes a hopper that stores a large number of pellets. These pellets funnel downwards, by gravity, towards a primary burn chamber. The burn chamber exposes the lowest pellets to an air flow, provided by an exhaust/heat tube. Pellets are lit in the primary burn chamber and burn until they reduce in size and fall through the rods or other members defining the burn chamber. A secondary burn chamber, typically a perforated plate, is located below the primary burn chamber and catches the partially burnt pellets, which continue to burn until they turn to ash, which falls through the perforations into an ash chamber below. As the pellets burn and fall from the primary burn chamber they are replaced by pellets falling through from the hopper.
For gravity-fed stoves, also referred to herein as natural draft stoves, manufacturers typically have no control over the amount of pellets that are in their primary burn chamber. Stove manufacturers typically regulate the heat output by controlling how much air passes over the burning pellets in the primary burn chamber. The amount of air movement in a natural draft pellet stove is very small when air passing over the pellets is restricted and the heat is reduced for a low temperature burn.
The low air movement of the natural draft is very fragile and it is easy to interrupt the draft, causing the pellets to smoke, smolder, and become extinguished. This creates challenges for doing a controlled and sustained low heat burn. With no control over the amount of burning pellets that are exposed to the moving air, most stove manufacturers rely on spacing in the primary burn chamber to regulate how fast the pellets drop into the primary burn chamber, burn down to a specific size, then drop down to the secondary burn plate where they finish burning to a specific size and drop down to the ash pan.
Using spacing to regulate how fast the pellets move through the primary burn chamber creates several challenges. A first challenge is that a stove that is calibrated with primary burn chamber spacing set to burn and drop a given amount of pellets to reach a given temperature on a high temperature burn at sea level will lose about 100 degrees of heat exhaust output when the same stove is moved to a 5,000 foot elevation. Also, such stove will burn much cooler due to the fact that there is less oxygen in the air.
A second challenge is created from the number of pellets that drop down onto the secondary burn plate. The secondary burn plate is typically a flat plate with holes and or slots cut in to allow the pellets to burn down to a much smaller size (hopefully ash) before dropping down to the ash pan. Pellets typically spend more time on the secondary burn plate than they do in the primary burn chamber. This creates a backup on the secondary burn plate and over a longer burn, the secondary burn plate backs up with unburned pellets. This backup of unburned pellets restricts the draft and/or plugs up the stove, so that the pellets begin to smoke, smolder, and eventually become extinguished.
If larger holes and/or slots are cut in the secondary burn plate, the pellets do not finish burning before dropping into the ash pan. Once the pellets drop into the ash pan they typically become extinguished due to the fact that there is less air movement and less heat in the ash pan. When the pellets fail to burn completely in the ash pan, the stove produces less heat, and the efficiency of using pellet fuel decreases.
An additional challenge comes from where the manufacturers of many natural draft gravity-fed stoves locate the secondary burn plate. The secondary burn plate is usually located about two inches below the primary burn chamber to create more space for pellet buildup on the secondary burn plate. If the secondary burn plate is located lower to allow for greater build up, the secondary plate is farther from the hottest part of the stove and is exposed to less air movement. This causes the pellets to burn more slowly, spending more time on the secondary burn plate. More time spent on the secondary burn plate contributes to pellet buildup which contributes to restricting the draft and/or clogging the stove, causing the pellets to smoke, smolder, and become extinguished.
What is therefore needed is an improved arrangement for enhancing the efficiency and control of a pellet stove.